[paul cotter (OP)]

Recently my mother in law, who is in her nineties and cognitively declining, said her microwave had failed accompanied by a loud bang. On enquiring what had happened she explained that she put some plates in to warm them up with nothing else in terms of a load! We explained that one cannot do that without risking damage but I was surprised that a one-off incident like that would kill it. Now I find out that she has been doing this repeatedly over a period of at least one year and possibly longer. I know thermionic devices will tolerate abuse but still for this length of time?

[alancalverd]

I don't recall seeing a "minimum load" specification for a domestic microwave. But if it has indeed warmed plates effectively, I guess the plates must have had some metallic content which could very well cause a bit of a bang. Or the failure may have been quite unconnected with the load - I have had a few explosive failures of induction cooker control units. 

[paul cotter (OP)]

It was not the plates that went bang, it was the internal circuitry. It did the same for me when I tried it with a loud bang typical of a capacitor discharging and the magnetron seemed to be partially functioning as it would heat a glass of water to a lukewarm but no higher. Without a load the magnetron would be subject to a very high vswr,  likely damaging the filament/cathode(grounded anode). In addition the anode would have to dissipate the entire energy input. This analysis could be flawed: I am very familiar with valves(vacuum tubes) and I am assuming magnetrons may be analysed in a similar fashion.

[vhfpmr]

I don't recall seeing a "minimum load" specification for a domestic microwave.

The handbook for every microwave I've ever owned says this:


Scan_20231222_0001.jpg (986.56 kB . 1650x2277 - viewed 727 times)

If you run it with no load there's nowhere for the power to go except back into the magnetron, where it doubles or triples its dissipation.

[alancalverd]

Memo to self (and PC's mother-in-law): RTFM!

I'm always dubious about the idea that thermionic devices are tolerant of abuse. In my experience they don't like being dropped, kicked, rattled about in a van or showered with beer, yet the good folk who sell guitar amplifiers seem to promote 500 watt kits with glass bottles and 20% efficiency, whilst the military specify transistors almost every time.

"Almost" because back in the 60's NATO dismantled a defected Sukhoi and discovered it was full of thermionic avionics, which are effectively tolerant of nuclear EMP and ionising radiation, unlike semiconductors.     

[paul cotter (OP)]

Thank you Alan for going to the bother of finding that leaflet and posting it. When I spoke of thermionic devices tolerating some degree of abuse I was referring to electrical abuse , not mechanical abuse. On the latter topic however, one can order(or could in the past) a ruggedized version of many of the common power devices for use in harsh and mobile environments, though they were considerably more expensive. PS: no overindulgence in the coming days, yours sincerely, the Grinch.

[Petrochemicals]

The plates do warm however, that is a load, Pyrex becomes very, very hot in the middle in no time what so ever, so that is a load after all if heating a plate did not work the good lady would surely not have repeated the action. I do not doubt though having a microwave running for hours at a time with little to absorb the energy does it any good. There is always the moisture in the air or the air itself, so the problem becomes what constitutes a load?

[evan_au]

I have some bowls with a blue glaze (perhaps based on cobalt?), which get quite hot in the microwave.
The glaze must absorb microwaves very effectively (and partially shield the food?)

[hamdani yusuf]

what constitutes a load

Objects that absorb the microwave radiation, and convert it to some other types of energy, instead of reflecting it back to the magnetron or passing it through, which will make it reflected by the oven wall, and end up back to the magnetron.
Those other types of energy could be heat, chemical energy, light, or radiation with different frequency which won't be reabsorbed by the magnetron.

[hamdani yusuf]

Here's a recent video from slow mo guy. Thanks to them, we can watch the exciting phenomena from a safe place.

Inside a Microwave at 80,000fps - The Slow Mo Guys

Gav and Dan pop some CDs in the microwave and watch the light show at a 3200th of real-time. Don't try at home.... for the sake of your microwave.

[SeanB]

Most common thing is that the reflected energy in the cavity will, due to the turntable, eventually return energy back into the probe end of the magnetron, leading to a high voltage flashover between the metal cap on the end and the waveguide cavity. This results in a burst of ionised air, and a dead short at RF of the magnetron output, with a loud bang. This RF short, with the stored energy in the magnetron cavities, and the capacitor between cathode and anode (the main capacitor outside has little effect, RF wise it is at the end of a really long RF trap by the cathode) results in the internal copper strapping inside the magnetron being subjected to very high currents, as the circulating energy is rapidly dumped via the output short, and those straps overheat and burn open circuit. This then results in the magnetron no longer being able to reach full power, even though it does draw the same current in operation, but the damaged cavities, like a whistle that has had the casing broken, will no longer oscillate with the circular flow of electrons from the hot cathode to anode.

You open the magnetron, and you will, after you saw through the end seal by the RF side, that there will be melted copper rings that strap the cavities together, so all cavities contribute to the RF output, even though only one has the output coupling loop in it.

https://upload.wikimedia.org/wikipedia/commons/1/16/Magnetron2.jpg

https://upload.wikimedia.org/wikipedia/commons/thumb/b/b0/Magnetron_section_transverse_to_axis.JPG/800px-Magnetron_section_transverse_to_axis.JPG

This shows the rings clearly, you will find them melted with the magnetrons that have had arcing over. the heavy copper strap that is leading up is the output loop, coupling off of the one cavity, and the rings around the cathode (silver in the middle) are the straps that connect the cavities, and also act to reduce harmonics as they strap alternate cavities together so that they will all resonate at the same 2.4GHz frequency.

[paul cotter (OP)]

Never cut a magnetron open, though I have in the past cut open some triodes and tetrodes(ones that were not suitable for rebuild) to find failure points.

[hamdani yusuf]

You can show image in this forum using insert image icon in preview page. Or flank the image link with tag [ img]and[/img]

You open the magnetron, and you will, after you saw through the end seal by the RF side, that there will be melted copper rings that strap the cavities together, so all cavities contribute to the RF output, even though only one has the output coupling loop in it.





This shows the rings clearly, you will find them melted with the magnetrons that have had arcing over. the heavy copper strap that is leading up is the output loop, coupling off of the one cavity, and the rings around the cathode (silver in the middle) are the straps that connect the cavities, and also act to reduce harmonics as they strap alternate cavities together so that they will all resonate at the same 2.4GHz frequency.

[vhfpmr]

Here's a recent video from slow mo guy. Thanks to them, we can watch the exciting phenomena from a safe place.

Inside a Microwave at 80,000fps - The Slow Mo Guys

Gav and Dan pop some CDs in the microwave and watch the light show at a 3200th of real-time. Don't try at home.... for the sake of your microwave.

Re the eggs in that video: I once made the mistake of assuming that they won't go pop without the shell.

[Zer0]

Imagine!

The Oven was equipped with a smart chip...
Which had access to your Browser History..
& Could also Call an anti-abuse helpline.

ps - i hope the blind rush of I.o.T does not lead to a Machine Uprising.
& if it ever happens, i wish They win!
:~]

[hamdani yusuf]

Imagine!

The Oven was equipped with a smart chip...
Which had access to your Browser History..
& Could also Call an anti-abuse helpline.

ps - i hope the blind rush of I.o.T does not lead to a Machine Uprising.
& if it ever happens, i wish They win!
:~]

How would it affect the seller/manufacturer?
How would it affect the buyer/user?

It would cost more to produce such a feature, which would increase price, and make the product less competitive.
If the users have intention to destroy their equipment, it's an opportunity for the manufacturer to sell more.

Economic view points to reject such a proposition. But if you are a curious billionaire not minding to lose money by subsidizing the price difference, it can happen.

[Zer0]

Imagine!

The Oven was equipped with a smart chip...
Which had access to your Browser History..
& Could also Call an anti-abuse helpline.

ps - i hope the blind rush of I.o.T does not lead to a Machine Uprising.
& if it ever happens, i wish They win!
:~]

How would it affect the seller/manufacturer?
It Won't.
In the Consumer Court they would present the Black Box Clause & say they never claimed to know how it worked, coz Nobody Knows!

How would it affect the buyer/user?
They will rise to Instant Fame which will begin as a Viral MeMe.
But they won't get a penny in Compensation for Damage to Reputation as they signed the Transfer of Ownership of the Product & Everything else joy or dispair that comes with it.

It would cost more to produce such a feature, which would increase price, and make the product less competitive.
The Next Best iPhone always costs More.
When one has a bit of a Limited Monopoly over the market, the Competitor matters Not!

If the users have intention to destroy their equipment, it's an opportunity for the manufacturer to sell more.
Imagine gifting a BonG to someone.
The User/addict has No Destructive Intentions.
But the same canNot be said for the Seller/peddler.

Economic view points to reject such a proposition. But if you are a curious billionaire not minding to lose money by subsidizing the price difference, it can happen.
Economics is related to Trade & Business.
A mutually beneficial co-existent relationship.
If the so called billionaire implants a chip inside your head for Free with No Hidden Costs n forces you to watch compulsory Adverts 100 minutes per day until you Subscribe to the Premium Version which will be ADs Free or a paid Critical System Update which comes at an Annual cost...then it ain't no Business nomore..that's Slavery!

ps - https://x.com/elonmusk/status/1625376580829847553?s=20

[hamdani yusuf]

Assuming that people will always make rational decisions can not be justified. Especially for those who haven't had a thorough thought about their own terminal goals.
That's why predictions in economy are often incorrect.